Parathyroid Scintigraphy - European Association of Nuclear Medicine

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cyclotron produced. Its uptake reflects amino acid reflux into stimulated parathyroid tissue (Otto et al. 2004; Beggs and Hain 2005). Uptake in inflammatory conditions may pose a problem and should be considered when interpreting images. The typical radioactivity dose ranges between 240 and 820 MBq, with an average intravenous dose of 400 MBq. 18 F-FDG 18 F-FDG has a half-life of 110 min and is cyclotron produced. 18 F-FDG allows glucose metabolism to be assessed and evaluated using PET. There is differential concentration of FDG in abnormal parathyroid tissue and this difference is used to demonstrate the abnormal gland. FDG also accumulates in other malignant and benign tissues, and in inflamed or infected tissue; this potentially limits its usefulness. The typical intravenous dose is 400 MBq. Radiopharmaceutical features Multiple radiopharmaceuticals have been described for the detection of parathyroid lesions. Thallium, sestamibi and tetrofosmin are the three most commonly used (Ahuja et al. 2004). All these agents were originally developed for cardiac scanning. In the 1980s, 201 Tl was the most commonly used agent, but it has a longer half-life and delivers a higher radiation dose to the patient (Kettle 2002). Consequently, 201 Tl is no longer commonly used, and most recent literature refers to the use of 99m Tc-sestamibi. However, for the subtraction 1 method it is probable that each radiopharmaceutical would provide the same diagnostic information (Kettle 2002). Subtraction agents Thyroid-specific imaging with 123 I or 99m Tcpertechnetate may be employed using a subtraction method to differentiate parathyroid from thyroid activity (Clark 2005). The two main agents used for imaging the thyroid are 123 I (sodium iodide) and 99m Tcpertechnetate. There is a slight preference for the use of 123 I, as it is organified and therefore provides a stable image. The pertechnetate washes out from the thyroid gland with time, and if there is some delay in imaging there may be a reduction in the quality of the thyroid image (Kettle 2002). However, both agents may be affected if the patient is taking thyroxine or anti-thyroid medications or has recently received iodine contrast agents. Thyroid-specific radiopharmaceuticals may aid delineation of the thyroid parenchyma if required after dual-phase imaging. This may be helpful as a second-line “visual subtraction” procedure when no parathyroid adenoma is visible on dual-phase parathyroid imaging (Clark 2005). Activities given for imaging the thyroid and parathyroid glands are as follows: 99m Tc pertechnetate, 80 MBq; 123 I, 40 MBq; 201 Tl, 80 MBq; 99m Tc-sestamibi, 900 MBq. If a 99m Tc-
pertechnetate/ 99m Tc-sestamibi combination is used then the radiation dose for the combined study is 11.6 mSv. If 123 I and 201 Tl are used, this rises to 18.3 mSv. Dual-phase agents 99m Tc-sestamibi and 99m Tc-tetrofosmin are commonly used agents for dual-phase parathyroid scintigraphy. The washout technique relies on the fact that while 99m Tc-sestamibi and 99m Tc-tetrofosmin are taken up by both the thyroid gland and the parathyroid at a similar rate, there is a faster rate of washout from the thyroid gland. These tracers localise in the thyroid gland as well as in parathyroid adenomas. This makes correlation of the adenoma in relation to the thyroid gland possible on planar as well as early SPECT imaging. 99m Tc-sestamibi is released from the thyroid with a half-life of about 30 min but is usually retained by abnormal parathyroid glands (Smith and Oates 2004). 99m Tc-tetrofosmin may clear more slowly from the thyroid gland. This differential washout improves the target-to-background ratio so that abnormal parathyroid tissue should be more visible on delayed images (Smith and Oates 2004; Clark 2005). However, thyroid adenomas and carcinomas can coexist and may retain 99m Tc-sestamibi or 99m Tc-tetrofosmin, resulting in false positive results (Smith and Oates 2004). 99m Tc-sestamibi and 99m Tc-tetrofosmin have comparable imaging characteristics. Usually, the choice of imaging agent depends on its 1 Chapter 2: Radiopharmaceuticals availability and the experience of the nuclear medicine radiologist. The dual-phase subtraction method with adjunctive thyroid-selective imaging ( 99m Tc or 123 I) may be helpful, or even essential, in patients with goitres or other confounding underlying thyroid disease, after thyroid surgery or in those patients with a palpable mass (Smith and Oates 2004). PET imaging agents Use of 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) and 11C- methionine PET for parathyroid imaging has been described (Otto et al. 2004; Beggs and Hain 2005). Initial studies with PET have shown conflicting results when using FDG as a tracer to image the parathyroid glands (Beggs and Hain 2005; Otto et al. 2004). It has been shown that 11C-methio nine PET holds more promise than FDG PET imaging of the parathyroid localisation (Beggs and Hain 2005). 11C-methionine PET scanning is of value in cases of primary hyperparathyroidism in which conventional imaging techniques have failed to localise the adenoma before proceeding to surgery, or in patients in whom surgery has been performed but has failed to correct the hyperparathyroidism (Beggs and Hain 2005). Adverse reactions to radiopharmaceuticals Table 2 shows side-effects and reactions to radiopharmaceuticals used for parathyroid scintigraphy. EANM
Page 1 and 2: European Association of Nuclear Med
Page 3 and 4: Contents Foreword 4 Sylviane Prévo
Page 5 and 6: Introduction Sue Huggett The first
Page 7 and 8: frequently (about 15% of cases) by
Page 9 and 10: et al. 1996; Hindié et al. 2000; M
Page 11 and 12: patients with renal failure (Hindi
Page 13: Radiopharmaceuticals Linda Tutty Ra
Page 17 and 18: 1 Chapter 2: Radiopharmaceuticals T
Page 19 and 20: y recording the time needed to acqu
Page 21 and 22: 1a) is defined by a fixed distance
Page 23 and 24: Obviously the maximum value of the
Page 25 and 26: • • • Language difficulties:
Page 27 and 28: Chapter 4: Patient preparation I ha
Page 29 and 30: Imaging protocols Nish Fernando and
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Page 33 and 34: Technical aspects of probe-guided s
Page 35 and 36: of preoperative scintigraphy is ver
Page 37 and 38: tamibi dose protocol, with a succes
Page 39 and 40: Chapter 6: Technical aspects of pro
Page 41 and 42: the role of surgery in the asymptom
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Parathyroid Scintigraphy - European Association of Nuclear Medicine

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